Pheromones are chemical signals secreted by one individual and received by another individual of the same species, in which they trigger a specific behavior or developmental process.

Animals use such chemical signals to communicate messages ranging from attraction to aggression and territorial marking.

One of the classical hallmarks of pheromone signaling is that both production of pheromones and the responses to them are gender specific. There are many well-documented cases of pheromonal communication in rodents and other mammals.

One of the hottest debates in the study of chemical senses is whether humans can produce and detect pheromones, and if so, whether they can use pheromone signaling to drive behavioral responses.

The strongest behavioral data supporting pheromone-based communication in humans have come from work on menstrual synchrony. It has been shown, for example, that female college roommates begin to menstruate at the same time.

Interestingly, this synchronization effect may be achieved solely by wiping underarm sweat from "donor" women onto the upper lips of "recipient" women, strongly suggesting that human pheromones may be contained in sweat.

But there has been little to no evidence to date directly assaying how it is that humans detect pheromonal signals. Furthermore, in other mammals there is a separate accessory olfactory system that is the primary pathway for processing pheromones, and there is only limited evidence that this pathway functions in adult humans.

Now Ivanka Savic and colleagues at Stockholm's Huddinge University Hospital have found definitive evidence for gender-specific activation of the hypothalamus (a region of the brain that is known to be involved in pheromone detection in rodents) in response to synthetic forms of steroid hormones.

The team took PET scans of people smelling compounds closely related to both testosterone and estrogen. PET scans measure the blood flow in different regions of a subject's brain.

They found that an estrogen-like compound produced increased blood flow in part of the hypothalamus in men, but not in women. Likewise, an androgen related to testosterone produced increased blood flow in the same part of the hypothalamus in women, but not in men.

This gender-specific activation of the hypothalamus is very different from the pattern of brain activation observed in response to ordinary odors.

These results, which convincingly demonstrate that the synthetic steroid hormones can act as pheromonal signals and that the human brain is capable of detecting them, are published in the August 30th issue of the journal Neuron.

In a related preview in the same issue of Neuron, Noam Sobel and Windy Brown of the University of California at Berkeley discuss the Savic results in light of what is known about pheromonal production and detection in other mammals, and outline the next important questions in the study of human pheromonal communication.